In recent years, studies on the optically compensated birefringence (OCB) cell that is to be used as a liquid crystal cell instead of a twisted nematic (TN) cell have been widely spread. In the OCB mode liquid crystal display, the liquid crystal molecules therein are aligned from homogenous state to splay state at the initial state when an external voltage is applied. However, when a higher voltage difference is applied to the OCB mode liquid crystal display, the liquid crystal molecules therein will transit from the splay state to the bend state, and it is required to spend some time for the transition above. In the bend state, the top and bottom liquid crystal molecules are always oriented symmetrically, and thus to compensate the birefringence of liquid crystal molecules so as to obtain the uniform viewing angle characteristic at all directions is more easily than that obtained with the orientation division method, and a high-speed response characteristic that is one order faster than that for the conventional TN cells may also be accomplished accordingly.
FIGS. 1A and 1B respectively illustrate the liquid crystal molecules in splay state and bend state in the OCB mode liquid crystal display device. As shown in FIG. 1A, in a splay state, the liquid crystal molecules 104 are uniformly splayed between the upper and lower substrates 100 and 102. However, when a voltage difference is applied to the glass substrates 100 and 102, the liquid crystal molecules 104 will transited into the bend state, as shown in FIG. 1B. In which, the transition time of the liquid crystal molecules 104 from the splay state to the bend state is one of the determinants for the OCB mode liquid crystal display device due to the fact that all the electro-optical properties of the OCB mode liquid crystal display device are operated when the liquid crystal molecules therein are in bend state.
However, some pixel structures was disclosed, such as what is disclosed in U.S. Pat. Nos. 6,115,087, 6,226,058, 6,661,491 and 6,597,424 but some of them are not suitable for OCB mode liquid crystal display devices, in addition a extra electrode existing in original pixel structures for transitioning the state of the liquid crystal was further disclosed in U.S. Pat. No. 6,933,916 and a auxiliary pier structure existing in the pixel structure with large voltage difference was disclosed in U.S. Pat. No. 6,801,284. However there still exist some demerits in those disclosed conventional pixel structures, for example, a space exists between the pixel electrode and the gate electrode, and the common electrode must be introduced and overlapped with the pixel electrode for a certain area so as to form a storage capacitor. However, the above two demerits will result in a small aperture which decrease the brightness and the contrast of the panel thereof.
In addition, although some driving methods for a liquid crystal display have been disclosed, such as that Takayuki Konno et al., (U.S. Pat. No. 6,873,377) and Katsuji Hattori et al., (U.S. Pat. No. 6,671,009) have disclosed a driving method for an OCB mode liquid crystal display and Hajime Nakamura et al., (U.S. Pat. No. 6,005,646) have disclosed another driving method for a thin film transistor liquid crystal display (TFT/LCD), there still exist some defects in the disclosed driving methods. For example, the driving method proposed by Katsuji Hattori et al. has a complicated signal input procedure and the applied system design always needs an alignment transition driving circuit, a switching control circuit and a switching circuit. In other words, the cost for the driving method of Katsuji Hattori et al. is always high and the relevant driving method is not so practical, especially for the trend of compactness. In addition, since the potential difference between the signal electrode and the common electrode is more than 10 volts and that between the gate electrode and the signal electrode is also more than 10 volts in the driving method proposed by Hajime Nakamura, there might exist some problems about the poor uniformity and the slow transition time in driving method of the prior arts.
As above, since all the electro-optical properties of the OCB mode liquid crystal display device are operated only when the liquid crystal molecules therein are in bend state, the liquid crystal molecules in the OCB mode liquid crystal display devices need to be transformed from the splay state (non-display state) into the bend state (display state) before being used and there still exist some demerits in the conventional pixel structures and driving methods, a new driving methods with excellent aperture ratio, sufficient brightness, clearly contrast and good uniformity for activating OCB mode liquid crystal display device are desired.